In all species, the class I MHC genes constitute a family of homologous DNA sequences, some of which encode the heavy chain moiety of the classical transplantation antigens. The functions of the remaining class I genes and their products are poorly, or not at all, understood. In the miniature swine, which contains only 7 class I homologous DNA sequences, two closely related genes encoding classical transplantation antigens have been isolated and characterized. A third sequence, which is highly divergent, has now also been isolated and is being characterized. By molecular hybridization to genomic DNA and direct DNA sequence analysis, it has been demonstrated that this gene, PD6, is a member of the limited class I SLA family. Despite extensive divergence, it appears to share the over-all sequence organization of a class I gene. The PD6 gene is expressed both in transfected mouse L cells and in vivo.